The demand for high-speed digital communications services, such as data, video, and high-definition TV, is growing. To accommodate these services transmission systems that operate in the multigigabit per second range are being developed with technologists concentrating on developing optical transmission systems because of their large bandwidth capabilities. Such high speed optical transmission systems require wide bandwidth receivers that are capable of receiving the optical signal and providing an electrical signal output. Transimpedance amplifiers are widely used in optical receiver applications as preamplifiers for converting received optical signals into an electrical signal output. However, the bandwidth performance of conventional transimpedance amplifier circuits is limited to a small fraction of the bandwidth of the embedded transistors because of the difficulty of achieving high gain at high frequencies.
To facilitate the discussion that follows, it is important to define two terms of art with regard to bandwidth performance. The first is the frequency where the signal response of the circuit or device drops by 3 dB below the mid-band frequency response; it is called the -3 dB bandwidth and is denoted as f.sub.-3db. The second is the frequency where a transistor produces unity gain (0 dB); it is called the unity gain frequency and is denoted by f.sub.t. For conventional transimpedance amplifiers, the f.sub.-3db is determined by the dominant pole defined by the relationship 1/(2.pi.RC), where R is the effective value of the load resistance and C is the effective value of the input capacitance at the active transistor's base. The f.sub.-3dB of the circuit cannot be any larger than the f.sub.t of the embedded transistors and, in application, is usually lower than f.sub.t by at least a factor A (the open loop voltage gain). Therefore, to improve the bandwidth capability of conventional transimpedance amplifiers, developers have had to try to minimize R or C, or improve the technology of the embedded transistors used in the circuit. However, reducing the value of R increases the circuit's internal noise and reduces the gain in the circuit (the gain is defined as i.times.R, where i is the photodiode current input at the base of the input transistor), thereby minimizing the effectiveness of the circuit's intended function.
U.S. Pat. No. 5,130,667, by G-K Chang, James L. Gimlett, and Ting-Ping Liu, entitled "Differential Transimpedance Amplifier", discloses a receiver amplifier that solves the problems identified above. The disclosed apparatus demonstrates that a circuit receiver front end can achieve high speed operation (greater than 10 Gb/s) and deliver the desirable differential output. It also has better noise performance at high bit rate operation because the noises from both the power supply lines and those picked up through RF antenna effect are common to both arms of the differential amplifier, thus the common mode noise rejection ratio can be high.
However, there is a second design objective for high speed optical receivers. Besides the need to be responsive to an input source operating at high frequencies, the receiver also must amplify the signal so that it has sufficient power to drive a data regeneration circuit at the next stage. The first step in that process is to increase the output voltage. In the Chang et al. patent the voltage at the output resistor is expressed by EQU V.sub.out1 =i.sub.photo .times.R.sub.1 EQU and EQU V.sub.out2 =V.sub.out1
where i.sub.photo is the photo current generated in the photodiode from incident optical power and R.sub.1 is the load resistor. Therefore, to increase the output voltage, the value of the R.sub.1 must be increased. However, because R.sub.1 is also analogous to the feedback resistor for an amplifier circuit, an increase in the value of R.sub.1 also increases the circuit gain thereby reducing the bandwidth capabilities of the circuit. Therefore, it is an objective of our invention to improve the frequency performance of the Chang et differential transimpedance amplifier circuit while also improving its gain.